The lambda rex gene, which maps adjacent to cI, is one of only two lambda genes known to be expressed in the lysogenic state, the other being cI itself. The lambda-specific function of the rex gene is unknown, since it can be defined only by the fact that lysogens for lambda rex plus prophage do not support the multiplication of T4rII mutants, whereas lysogens for lambda rex phage allow near-normal T4rII multiplication. Experiments in several laboratories indicate that cI and rex are regulated co-ordinately, suggesting that rex- protein may perform an important control function in the decision to lysogenize. Yet rex-mutants seem to be unaffected in vegetative multiplication of lambda or in the establishment or maintenance of lysogeny. The purpose of the proposed research is two-fold: First, to determine the function of rex either phenomenologically by defining conditions in which rex function is essential to lambda or biochemically by attempting to isolate isotopically pure rex protein from mixed extracts of UV-irradiated differentially (C14- and H3-) labeled cells infected with rex plus and rex minus phage; if isotopic purification is successful, the purification procedure will be used as a basis for concentrating the protein sufficiently to permit the elucidation of its lambda-specific function. The second purpose is to use a newly-isolated mutant defective in the promotor (prm) for transcription of the cI-rex messenger RNA to investigate the relationship to prm and pre in the synthesis of cI-rex mRNA during the establishment of lysogeny.